In a top-irradiation type vehicle inspection system, a radiation source is provided on the top of an inspection passage, a detector is provided on the bottom of the inspection passage. The inspected vehicle is typically driven to advance by chain type dragging means. The dragging means includes plate like carrier frame on the bottom, chain and pushing roller on the frame. In the power of motor, the inspected vehicle is moved by means of chain and pushing roller. Since supporting plates of the carrier frame is a metal structural, thus shielding ray beams when the vehicle is inspected. Consequently, the detector fails to obtain singles and meanwhile the obtained image is black, which severely influences the image quality and the inspection effect.